Substance use disorders are a pervasive health problem for homeless persons. Among this population, substance use disorders not only create psychological suffering and a serious public health burden;they also create additional barriers to recovery from homelessness. Some research indicates that the development of healthier social networks might underlie homeless persons'long-term recovery from substance abuse and broader functional outcomes. However, evaluating the mediational role of social network changes requires the collection of longitudinal data, which is extremely difficult with this population. With private funding from St. Francis House (SFH), the largest day-service provider for homeless individuals in New England, the Division on Addictions has taken an important step toward overcoming common obstacles to longitudinal research on homelessness. More specifically, the Division has created and implemented new computer software that enables the collection of longitudinal data on SFH guests as they access a wide range of SFH programs and services. However, private funding is limited to technology development and does not support research activities, and SFH staff lack the scientific training and experience to conduct sophisticated research projects. Therefore, we seek Support Opportunity for Addiction Research (SOAR) support to use the new computer software to conduct a systematic program evaluation and to test a model specifying potential social network mechanisms of change among this population. We will accomplish these Specific Aims by conducting a longitudinal study of substance abuse recovery mechanisms among homeless adults. We will use structural equation modeling to test the mediating role of changes in social network quality and structure in producing functional improvements. The study will draw upon data from approximately 144 SFH guests over a data- collection period of six months. Results will have theoretical implications as well as practical significance beyond SFH. PUBLIC HEALTH RELEVANCE: People with substance abuse are overrepresented among those who experience homelessness in the United States. Beyond creating psychological suffering and a substantial public health burden, substance use disorders create barriers to homeless persons'self-sustainability. The proposed research will seek to uncover mechanisms that underlie homeless persons'long-term recovery from substance abuse.